The present disclosure relates to magnetic resonance (MR) imaging. More particularly, the present disclosure relates to methods of estimating and mapping the main magnetic field (B0), where B0 inhomogeneity is introduced when a body is placed in a MR scanner.
An accurate B0 off-resonance map has many applications, such as in shimming, image distortion correction, quantitative susceptibility mapping and fat/water separation. However, accurate estimation of the B0 map can be hampered by the additional phase term related to the chemical shift of fat (1). To avoid the effect of chemical shift, conventional B0 mapping is performed by phase unwrapping of the complex data where signals from fat and water are “in-phase”, which is ensured by using a single-peak model for both water and fat, e.g. 3-point Dixon technique (2,3).
Recently, multi-echo gradient echo (GRE) acquisition has attracted the attention of the academic community because of its potential for performing R2* (decay rate) mapping, B0 mapping, quantitative fat/water separation and quantitative susceptibility mapping from a single scan. Many multi-point Dixon techniques (1,4,5) have been established to quantify fat-fraction (FF) through iterative multi-parameter fitting procedures by using a multi-peak fat model, where the fitting parameters include the frequency shift (fb0) due to B0 inhomogeneity, the signals of the water (ρW) and fat (ρF) components, and R2*. The results of the iterative procedure on a voxel-by-voxel basis (6) can be further enhanced using region-growing (7), region-merging (8), region-labeling (9), fat likelihood analysis (10), applying a multi-resolution strategy for guiding the search process (11,12), and algebraic decomposition (13). The state of the art is to estimate the aforementioned multi-parameters jointly for all voxels by using the graph-cut algorithm in B0 map estimation (4, 14-16).
The multi-point Dixon techniques mainly aim to generate an accurate FF map instead of a realistic and accurate B0 map. To improve the robustness for preventing fat/water swaps, most of these techniques apply two constraints during the B0 searching process: the prior knowledge of spatial smoothness and a limited range of fb0 values. Because the effect of the phase term in the complex signal equation is periodic, an accurate FF measurement can be achieved from either “non-smooth” (9) or “over smooth” (8) B0 maps. Further, if the true B0 inhomogeneity is outside the range, the resulting B0 map will be wrapped and might not be useful for further data processing.